Colorectal cancer (CRC) is the third most common cancer in women and the fourth in men [
1]. Little over 1.2 million cases are diagnosed each year globally with about 600,000 deaths. The primary cause of colon cancer induced death is due to metastasis to the liver [
2]. Nearly, 50 % of the patients diagnosed with colorectal cancer show tumor recurrence, which is assumed to be due to the presence of chemotherapy-resistant cancer stem cells (CSCs) [
3]. Therefore, newer treatment strategies are urgently needed for reducing the rate of recurrence and thereby improving the overall survival of patients diagnosed with colorectal cancer. We have focused our investigation to finding ways to restore the expression of specific microRNAs (miRNAs) that are down-regulated in colorectal cancer and are involved in the progression of this malignancy.
The miRNAs are a class of endogenous small non-coding RNAs that control gene expression through binding to the seed sequence at the 3´-UTR of target mRNAs, resulting in translational repression or mRNA degradation [
4]. It has been predicted that over 30 % of the human protein coding genes are post-transcriptionally regulated by this mechanism [
5]. miRNAs have also been shown to regulate numerous processes of carcinogenesis, including the growth and maintenance of cancer stem-like cells (CSLCs) which are known to be resistant to chemotherapy and possess the limitless capacity to regenerate [
6,
7]. The CSLCs play critical roles in the development and progression of many malignancies, including colorectal cancer [
8]. Family of miR-34 that includes 34a, b and c has been reported to inhibit CSLCs [
9]. They are down-regulated in colorectal cancer [
10] which may contribute to the progression of the disease as well as drug resistance[
11]. Emerging evidence suggests that p53 acts as a transcription factor to increase the expression of the miR-34 family members which, in turn, modulate cell cycle progression, senescence and apoptosis, inhibition of invasion and migration [
12,
13]. Interestingly a positive feedback loop exists between p53 and miR-34a [
14]. The p53-induced expression of miR-34a inhibits its target gene SIRT1, a histone deacetylase. Down-regulation of SIRT1 expression up-regulates p53 acetylation and the transcriptional activity of p53 [
15]. Indeed, up-regulation of miR-34 has been shown to induce cell-cycle arrest, inhibition of invasion and migration and p53 induced apoptosis [
16,
17]. In view of this, it is tempting to speculate that p53-mediated processes of apoptosis in colon cancer cells could be affected by down-regulation of miR-34. However, little is known whether agent(s) that modulates colon CSLCs would also modulate the family of miR-34 in colon cancer cells or not.
In search of such agents, we tested the effects of our recently generated difluorinated curcumin (CDF), a novel analog of the dietary ingredient curcumin, with much greater bioavailability than the parent compound [
18,
19]. Recent data from our laboratory suggest that CDF´s anti-tumor activity is mediated by multiple mechanisms including regulation through miRNAs [
20‐
23]. CDF also causes a marked inhibition of cellular growth and induces apoptosis in chemo-resistant (5-Fluorouracil and Oxaliplatin-resistant) colon cancer cells, and demonstrates a remarkable ability to disintegrate colonospheres [
24] that are considered to be surrogate tumors [
25]. Suffice to mention that the chemo-resistant colon cancer cells and colonospheres are highly enriched in CSLCs [
25,
26]. This relevant information prompted us to determine whether CDF could be utilized to modulate the family of miR-34, and if so, whether CDF-induced modulation of miR-34 could in part be attributed to epigenetic alterations, specifically the methylation status of the promoter of miR-34.